Various types of biometric systems are used more and more in order to provide an increased security for accessing an electronic device and at the same time keep the user convenience at an acceptable level. In particular fingerprint sensors have been successfully integrated in such devices, for example, thanks to their small form factor, high performance and user acceptance. Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important.
All capacitive fingerprint sensors provide an indicative measure of the capacitance between several sensing elements and a finger placed on the surface of the fingerprint sensor. Acquisition of a fingerprint image is typically performed using a fingerprint sensor comprising a plurality of sensing elements arranged in a two-dimensional manner, and a block based technique may be applied to the fingerprint sensor for acquiring a fingerprint image, where the blocks of sensing elements are sampled sequentially.
One of the problems associated with fingerprint sensors concerns the reliable and accurate transformation of the ridge-valley pattern of the fingertip into electrical signals. That is, the image quality achieved by the fingerprint sensor, in particular contrast and brightness, depends on how good the connection is to the image which brings about the measured changes in capacitance, in particular—in the case of a fingerprint sensor which is principally of interest here—how firmly a fingertip is placed on to the bearing area of the fingerprint sensor. If the finger is only placed lightly thereon, then a fingerprint image is produced which has a small area and only a small amount of dark gray components. If the finger is pressed firmly on to the sensor, then the area of the fingerprint image is increased and the gray components in the image are shifted toward darker values. When the finger reaches a short distance from the fingerprint sensor during placement onto the surface of the fingerprint sensor, an as yet incomplete fingerprint image can already be generated since the capacitance of the conductor areas that is measured with respect to the skin surface can already make up a difference with respect to the basic state of the fingerprint sensor.
There is thus of high importance to be able to acquire a fingerprint image at “the right time” when the finger is placed onto the fingerprint sensor. An exemplary implementation for trying to overcome this problem is disclosed in U.S. Pat. No. 6,990,218. U.S. Pat. No. 6,990,218 provides a method adapted to select an image that is best suited to further processing from a sequence of images that are recorded in short succession one after the other by the sensor. The image is selected based on the criteria that it is the image that most likely corresponds to a typical fingerprint. The selected image is further improved by subtracting a correction image from the selected image, where the correction image is essentially formed to represent background disturbances when acquiring the sequence of images.
Even though U.S. Pat. No. 6,990,218 introduces an interesting approach to improving the timing when acquiring a fingerprint image as well as for reducing disturbances within the image, U.S. Pat. No. 6,990,218 discards possibly valuable information. Thus, there appears to be room for further improvement in regards to fingerprint image acquisition during placement of a finger onto a fingerprint sensor.